So, Sejuani just got a big rework! You're just in time!
Her new passive doubles her Armor and Magic Resistance, and adds 100 to it, while also making her immune to slows, but only for a few seconds; it goes away a short bit after she takes damage.
Her Q is the same.
Her W is a two-part skillshot that strikes out with her flail in the target direction, first in a cone, and then in a line.
Her E is interesting. She gains the power to stack 'Frost' on enemies with her AAs and with hits of her W, and her melee allies close to her can also stack the Frost for her. Once someone has 4 stacks, she can target them with a point-click stun. Attacking them shatters them, dealing a large amount of Magic Damage. The same target can't be stunned in short succession, but the ability has a very short cooldown, so stacking Frost on many people allows her to stun a lot of enemies.
Her ult only stuns one target now, the target struck, but if it flies more than half of it's range, it summons an ice storm that slows enemies before collapsing for extra damage and a stronger slow.
She's great, try her out.